Science: What sends a lemming over the edge?: Malcolm Smith reports on a likely explanation for the mass suicides of these northern rodents

Devotees of the computer game 'Lemmings' know how difficult it is to ensure that these stubby-tailed rodents arrive safely at their destination. In the wild - in Scandinavia and north-west Russia - it is impossible.

These little rodents, no more than five inches in length, undergo one of the best-known cyclical fluctuations in their numbers that nature ever invented. Wildlife documentary after wildlife documentary has shown close-up footage of thousands of the little, vole-like mammals careering headlong into rivers and the sea - frequently to certain death. The question is, why?

Lemmings are cold-climate mammals. They live in moist, stony, tundra vegetation consisting of sedges, willow scrub and dwarf birch. For half the year - often longer - their environment is frozen and snow-covered.

Northern Europe and north-west Asia have three species, of which the best-known is the Norway lemming - the film star. Usually nocturnal, it has a stumpy tail, short legs, and a round body covered in thick, yellow-brown fur patterned with dark brown streaks and patches.

Every three to five years, lemming numbers peak, then suddenly decline again. At these population peaks, the excess number of rodents often move down from their mountain tundra homes to valleys and spread out, sometimes en masse. Eventually, some reach the sea and attempt to continue this frenzied dispersal. With uncanny timing, the wildlife film-makers are usually on the spot.

All sorts of theories to explain these enormous population fluctuations have been put forward. They include predation, nutrition, disease, parasites, competition and genetic changes. None has been proven. Another theory - that lemming population swings are related to chemical changes in the plants they eat - had not been tested - until recently.

Tarald Seldal and Goran Hogstedt of the University of Bergen, and Knut-Jan Andersen of Haukeland Hospital in Bergen, have done just that. Their findings provide very strong circumstantial evidence that chemicals produced by the plants grazed by Norway lemmings control the ups and downs these rodents have to cope with.

Lemmings graze plants like stiff sedge and cotton grass. Both are abundant in the Scandinavian tundra, and both produce defensive chemicals when damaged. The heavier the grazing - in other words the damage - the more of these chemicals they produce. Tundra plants are not unique in this; many plants produce such defensive substances.

Some of these chemicals are proteins which inhibit the activity of proteases (enzymes which break down protein in food) in the intestines of mammals. The most important feature of these inhibitors is that they put a stop to the action of trypsin, a protease secreted by the pancreas.

Grazing mammals ingesting these inhibitors can't digest proteins in their food. To make matters worse, reduced protein digestion stimulates more production - by the pancreas - of proteins which, in turn, are inhibited and lost in the faeces. These enzymes contain large amounts of essential dietary amino acids which are rare in the plants eaten. So lemmings and other grazing mammals, though they may eat voraciously, slowly starve. An enlarged pancreas is an associated symptom.

The Norwegian researchers found that levels of the inhibitor in these plants rose considerably in known years of peak lemming numbers and remained high during the following year of their decline, falling back to a very low level in the next year when lemming numbers again returned to a population low.

Lemmings taken from a declining population had pancreases nearly three times the normal size. They also had retarded growth, 25 per cent of them dying within 10 days of capture despite consuming more than 10 times their own body weight in food each day.

It is probable that the high trypsin inhibitor levels in peak and early decline lemming years can also explain the rodents' delayed sexual maturation, compressed breeding season and high dispersal rates at such times.

To cause the cyclical changes in lemming numbers, there has to be a link between high rodent numbers and the quantity of the inhibitor produced. There is, and it is related to grazing densities.

Cotton grasses and sedges grazed very occasionally produced inhibitors for only a short period of time, peaking around 30 hours after each bit of damage. But, if lemming numbers are high, and the plants are frequently grazed, more inhibitor is produced and it stays at high concentrations for as long as the grazing continues.

So low densities of lemmings can eat, digest their proteins and reproduce to their hearts' content. But once their populations start to climb, as inevitably they do, their food plants take defensive action. Mass starvation, and a frenzied search for pastures new, is the result.

If it is any consolation, the roller-coaster of lemming control at least guarantees that there will always be plants for them to live on. But, you might well ask, isn't it a wonder that evolution hasn't seen to it that the plant and the plant-eater reach a more stable accommodation? Of course not, otherwise what would the wildlife film-makers do with themselves?

(Photograph omitted)

News
Patrick Stewart in the classiest ice bucket to date
peopleSir Patrick took a more understated approach to the challenge
News
The current recommendation from Britain's Chief Medical Officer, is that people refrain from drinking on at least two days a week
scienceTheory is that hangovers are caused by methanol poisoning
Arts and Entertainment
tvWe have created an infogaphic that looks back over the previous incarnations of the Doctor
Sport
Olivier Giroud celebrates after his late goal saved Arsenal a point at Goodison Park
football Giroud rescues a point for Arsenal after they trailed by two goals
PROMOTED VIDEO
News
i100
News
newsComedian Lee Hurst started trend with first tweet using the hashtag
News
i100
News
Australian rapper Iggy Azalea was left red faced but, thankfully, unhurt after taking a few too many steps backwards, sending her tumbling off the stage.
people
Life and Style
A nearly completed RoboThespian robot inside the Engineered Arts workshop is tested in Penryn, England. The Cornish company, operating from an industrial unit near Falmouth, is the world's only maker of commercially available life sized humanoid robots
techSuper-intelligent robots could decide destroying the human race is the kindest thing to do
Life and Style
techConcept would see planes coated in layer of micro-sensors and able to sense wear and tear
News
ebooksAn evocation of the conflict through the eyes of those who lived through it
News
scienceExcitement from alien hunters at 'evidence' of extraterrestrial life
News
newsRyan Crighton goes in search of the capo dei capi
Life and Style
Customers can get their caffeine fix on the move
food + drink
News
i100
Extras
indybest

Independent
Travel Shop
the manor
Up to 70% off luxury travel
on city breaks Find out more
santorini
Up to 70% off luxury travel
on chic beach resorts Find out more
sardina foodie
Up to 70% off luxury travel
on country retreats Find out more
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Software Developer (Java /C# Programmer)- London

£30000 - £45000 per annum: Harrington Starr: A global investment management fi...

Senior Network Engineer-(CCIE, CCNP, Cisco, London)

£65000 - £75000 per annum: Harrington Starr: Senior Network Engineer-(CCIE, CC...

Senior Network Analyst - (CCIE, Cisco, CISSP)

£70000 - £80000 per annum: Harrington Starr: Senior Network Analyst - (CCIE, C...

Senior Network Engineer-(Design, Implementation, CCIE)

£60000 - £80000 per annum: Harrington Starr: Senior Network Engineer-(Design, ...

Day In a Page

All this talk of an ‘apocalyptic’ threat is simply childish

Robert Fisk: All this talk of an ‘apocalyptic’ threat is simply childish

Chuck Hagel and Martin Dempsey were pure Hollywood. They only needed Tom Cruise
Mafia Dons: is the Camorra in control of the Granite City?

Mafia Dons: is the Camorra in control of the Granite City?

So claims an EU report which points to the Italian Mob’s alleged grip on everything from public works to property
Emmys look set to overhaul the Oscars as Hollywood’s prize draw

Emmys look set to overhaul the Oscars as Hollywood’s prize draw

Once the poor relation, the awards show now has the top stars and boasts the best drama
What happens to African migrants once they land in Italy during the summer?

What happens to migrants once they land in Italy?

Memphis Barker follows their trail through southern Europe
French connection: After 1,300 years, there’s a bridge to Mont Saint-Michel

French connection: After 1,300 years, there’s a bridge to Mont Saint-Michel

The ugly causeway is being dismantled, an elegant connection erected in its place. So everyone’s happy, right?
Frank Mugisha: Uganda's most outspoken gay rights activist on changing people's attitudes, coming out, and the threat of being attacked

Frank Mugisha: 'Coming out was a gradual process '

Uganda's most outspoken gay rights activist on changing people's attitudes, coming out, and the threat of being attacked
Radio 1 to hire 'YouTube-famous' vloggers to broadcast online

Radio 1’s new top ten

The ‘vloggers’ signed up to find twentysomething audience
David Abraham: Big ideas for the small screen

David Abraham: Big ideas for the small screen

A blistering attack on US influence on British television has lifted the savvy head of Channel 4 out of the shadows
Florence Knight's perfect picnic: Make the most of summer's last Bank Holiday weekend

Florence Knight's perfect picnic

Polpetto's head chef shares her favourite recipes from Iced Earl Grey tea to baked peaches, mascarpone & brown sugar meringues...
Horst P Horst: The fashion photography genius who inspired Madonna comes to the V&A

Horst P Horst comes to the V&A

The London's museum has delved into its archives to stage a far-reaching retrospective celebrating the photographer's six decades of creativity
Mark Hix recipes: Try our chef's summery soups for a real seasonal refresher

Mark Hix's summery soups

Soup isn’t just about comforting broths and steaming hot bowls...
Tim Sherwood column: 'It started as a three-horse race but turned into the Grand National'

Tim Sherwood column

I would have taken the Crystal Palace job if I’d been offered it soon after my interview... but the whole process dragged on so I had to pull out
Eden Hazard: Young, gifted... not yet perfect

Eden Hazard: Young, gifted... not yet perfect

Eden Hazard admits he is still below the level of Ronaldo and Messi but, after a breakthrough season, is ready to thrill Chelsea’s fans
Tim Howard: I’m an old dog. I don’t get too excited

Tim Howard: I’m an old dog. I don’t get too excited

The Everton and US goalkeeper was such a star at the World Cup that the President phoned to congratulate him... not that he knows what the fuss is all about
Match of the Day at 50: Show reminds us that even the most revered BBC institution may have a finite lifespan – thanks to the opposition

Tom Peck on Match of the Day at 50

The show reminds us that even the most revered BBC institution may have a finite lifespan – thanks to the opposition